Return errno directly with seccomp filters

[quantum5]

libseccomp's seccomp_rule_add supports an action called SCMP_ACT_ERRNO(errno), so all our handlers that generates errnos could theoretically be handled in seccomp.

[Xyene]

I considered this before, but I don't think we have a good enough ptbox testing story currently. I'd be worried about the ptrace-only errno implementations bitrotting unnoticed.

[Xyene]

Given #871 is in the pipeline, we can commit to dropping the ptrace-only backend and go ahead with this.